The potential reserves of liquid hydrocarbons contained in subterranean carbonaceous deposits are known to be very substantial and form a large portion of the known energy reserves in the world. In fact, the potential reserves of liquid hydrocarbons to be derived from oil shale greatly exceed the known reserves of liquid hydrocarbons to be derived from petroleum. As a result of the increasing demand for light hydrocarbon fractions, there is much current interest in economical methods for improving the recovery of hydrocarbon liquids from oil shale on commercial scales.
It has long been known that oil may be extracted by retorting from various extensive deposits of porous minerals known by their generic term "oil shale", which are permeated by a complex organic material called "kerogen". Upon application of retorting, the kerogen is converted to a complex mixture of hydrocarbons and hydrocarbon derivatives which may be recovered from a retort as a liquid shale oil product. While retorting may be the most common method utilized to recover hydrocarbon fluids from oil shale, it has several disadvantages one of which is that shale oil cracks to gas readily at conventional retorting conditions. The cracking of shale oil to gas is disadvantageous in that it substantially reduces the total oil recovered from the oil shale.
Furthermore, retorting is not very successful on all types of oil shales. For example, Eastern shales are known to contain an equal proportion of organic carbon as the Western shales. However, upon retorting, only about 30 percent of this carbon is converted to oil. This conversion is less than half of the conversion achieved by retorting Western shale. To clarify this fact, consider two oil shale samples containing 13.6 percent organic carbon. Retorting the Western shale would reduce this carbon to about four percent. On the other hand, retorting Eastern shale would reduce this carbon to only about 10 percent. Thus, any technique that may be used to improve this conversion as measured by enhancement in oil yield will be highly advantageous particularly when applied to Eastern shale.
Accordingly, the present invention provides a process to enhance the yield of hydrocarbon fluids from oil shale by treating the shale under milder conditions than retorting conditions.
U.S. Pat. No. 4,238,315 to Patzer, II, relates to a process for recovering oil from oil shale containing kerogen which comprises bringing a mixture of oil shale and solvent to a temperature in the range of about 385.degree. to about 400.degree. C. in a time period of less than about 10 minutes, maintaining the mixture at a temperature in the range of about 385.degree. to about 440.degree. C. and a pressure in the range of about 250 to about 2,000 psig for a period of about 20 minutes to about 2 hours and thereafter recovering the resulting oil. These conditions are much more severe than those utilized in the present invention. Furthermore, Patzer states that a weight ratio of solvent to shale of at least 1.25:1, preferably at least 1.5:1 must be employed. This is a very high ratio of solvent particularly when one considers solvent cost, increased heating costs, capacity requirements of equipment, and storage facilities in plants.
U.S. Pat. No. 4,325,803 to Green et al relates to a method for the separation and recovery of organic material from rock which includes forming a slurry comprising rock containing organic material and a hydrogen transfer agent that is liquid at standard conditions, subjecting the slurry to elevated temperatures (300.degree. to 650.degree. C.) and elevated pressures (10 atmospheres to 200 atmospheres), and subjecting the product to adiabatic flash vaporization. The required conditions of the Green et al process are again much more severe than those utilized in the present invention. The Green et al process not only requires that the amount of hydrocarbon liquid added to the shale be at least 25 weight percent of the shale, but also requires that the hydrocarbon liquid contain at least 25% hydrogen donating compounds. Furthermore, the Green et al process is limited to utilizing hydrogen transfer liquids which have a low boiling point not greater than 325.degree. C. (617.degree. F.). Thus, not only is the amount of solvent required excessive but the solvent is limited to lighter cuts with the additional requirement that the lighter cuts contain at least 25% hydrogen donating compounds.
Hampton in U.S. Pat. No. 1,778,515 states that it is old to subject a bituminiferous material, such as oil shale, to the digestive action of an oil bath to recover oil from oil shale. It is further stated that increased yields of oil can be obtained by mixing oil shale of 11/2 inch mesh with a heavy oil, which may be preheated, heating the resulting mixture gradually to a temperature of 300.degree. to 400.degree. F. (144.degree. to 204.degree. C.), grinding the shale in the heated mixture until 60 percent or more thereof will pass 200 mesh, and then heating the ground mixture, most desirable suddenly, to a materially high temperature in the range of about 600.degree. to about 700.degree. F. (316.degree. to about 371.degree. C.). Hampton considers the possibility of feeding dry pulverized shale, without any accompanying oil, in controllable amounts into a hot digestion bath, but advises against the same because of technical difficulties.